Homeostatic changes during post-nap sleep maintain baseline levels of delta EEG.

It has been hypothesized that visually scored stage 4 EEG (dense, high amplitude 0.5-3 Hz (delta) waves) is a correlate of a metabolic process that reverses some of the effects of waking on the brain. The results of nap studies appear inconsistent with this hypothesis since late naps produce a disproportionate loss of stage 4 during subsequent sleep. We show here with direct computer measurement that the integrated amplitude (and other measures) of 0.5-3 Hz EEG waves are conserved across a nap and post-nap sleep. Thus, the metabolic model remains tenable. However, the homeostatic adjustments involve changes in the periods, durations and distributions of delta waves that are not predictable by any existing model. This study also demonstrates the limitations inherent in visual estimates (sleep stage scoring) of delta wave amplitude and abundance.